File this one under "Mick Being Mick," "Of Course He Did" or just "Ewww."

Katy Perry is spilling a big celebrity secret from her past: She was hit on by Mick Jagger when she was just 18 years old! The Rolling Stones legend made moves on the young singer despite their 40-year age difference. We repeat: 40-YEAR AGE DIFFERENCE.

The Rock Star's Rock Star is a notorious womanizer, so some will say, "Hey, at least she was 18!" In his youth, Mick was a Harry Styles lookalike ... but that was a loooong time ago, even when this all went down (Jagger turned 70 this year).

Katy told Aussie radio station NovaFM that the then 60-something Jagger hit on her when she sang backing vocals for him in 2004. Fittingly, considering Mick's love of the ladies, the song was called 'Old Habits Die Hard.'

"I actually went to dinner with him one time," Katy dished. "And he hit on me when I was like 18!"

How did she get out of the sticky situation? "Well, you bring a friend, and then they do them," she said. "You sacrifice your friend." Um, let's just agree to hope that it isn't true.

Katy says that she and Mick have kept things courteous -- with her even appearing onstage with the Stones -- and that Mick has "been very kind."

Hey, they don't call it 'Moves Like Jagger' for nothing!